1. Field of the Invention
This invention relates to a process for the low-waste production of sodium dichromate from the mineral chromite with simultaneous recovery of low-carbon ferrochromium.
Of the various minerals which contain chromium, only the chromium spinels, particularly chromite (chrome ironstone, idealized: FeCr.sub.2 O.sub.4), are of economic importance.
Sodium dichromate is by far the most important starting material for the production of chromium compounds.
Accordingly, the conversion of chromite into sodium dichromate is the crucial step from the minerals to the chromium chemicals with their broad range of applications.
2. Description of the Prior Art
The only process carried out on an industrial scale is based on the alkaline digestion of chromite with soda or sodium hydroxide and air in the presence of a leaning agent. This process which is described in detail elsewhere is attended by two serious disadvantages, namely:
a) The chromium is never completely dissolved out, the yield reaching just under 90% of the original content of Cr.sub.2 O.sub.3 ; 4 to 6% Cr.sub.2 O.sub.3 remain in the residue. PA1 b) The elements accompanying the chromium as oxides and hydroxides form the residue which has to be discarded and disposed of. The principal constituent of the residue is iron oxide although it cannot be put to any further use in this impure form. The chromate obstinately remaining therein has to be rendered inert by an aftertreatment. PA1 c) The digestion process is very slow so that the starting materials have to be very finely ground to achieve economically acceptable reaction times. PA1 d) The excess of alkaline digesting agent, for example sodium carbonate or sodium hydroxide, has to be limited to minimize digestion of the approximately 5 to 28% aluminum oxide present in the chromite. The quantities of valuable alkali used for the digestion of aluminium oxide are not only lost, their presence in the sodium chromate solution produced from the digested material can also lead to serious problems in the further course of the process. The aluminate passing into solution despite the limitation of the quantities of alkaline digesting agent has to be precipitated with acidic agents, preferably with dichromate solution, during the actual dissolution process. PA1 oxidizing digestion of chromium ore or chromium ore concentrate under alkaline conditions PA1 leaching out the sodium chromate formed PA1 converting the sodium monochromate into dichromate by acidification of the solution. PA1 passed through the furnace, PA1 expensively separated and PA1 finally aftertreated and disposed of as fine-particle, reactive, oxidized water-rich sludge containing residual chromium. PA1 1. would enable the chromium present in the chromite to be almost completely reacted, PA1 2. would reduce the input of accompanying elements into the digestion process and, hence, reduce or even totally avoid the amount of residue for disposal, PA1 3. and at the same time would put the iron present in the chromite to some use, PA1 4. would largely reduce or even totally eliminate the expense involved in keeping troublesome impurities, such as aluminium, away from the alkaline sodium chromate solution. PA1 A slag which contains the secondary constituents MgO and Al.sub.2 O.sub.3 present in the chromite and which may be conditioned for use as building materials or fillers either as such or by addition of Ca-containing substances or quartzite. PA1 The second phase is the metallic phase, the ferrochromium, which contains the largest part of the chromium and iron bound in the spinel. In addition, the ferrochromium contains residual carbon (5 to 7%) and fractions of Si through reduction of silicates present in the ore. PA1 a slag containing increased levels of oxidized chromium PA1 and a low-carbon residual metal phase which is free from silicon and which consists: essentially of iron and the non-oxidized chromium (low- or medium-carbon ferrochromium). PA1 blowing the oxygen onto the melt with a lance PA1 injecting the oxygen into the melt through nozzles arranged at the side or at the bottom of the vessel PA1 or by a combination of both methods. PA1 In both cases, the oxygen may be mixed with fuel gases or inert gases. PA1 1. The separation of Cr and Fe in the converter by freshening with oxygen enables chromium oxide slag with high Cr.sub.2 O.sub.3 contents of up to 100% to be produced. PA1 2. Through the use of a ferrochromium melt as starting material for a synthetic raw material for the production of sodium chromate, the input of the inert constituents MgO, Al.sub.2 O.sub.3, FeO and Fe.sub.2 O.sub.3 into the oxidizing alkaline digestion process is considerably reduced. PA1 3. In the production of chromium oxide slag by the described method, the iron does not accumulate as oxide made worthless by impurities, but instead in the form of low-carbon ferrochromium. This alloy is in great demand as a raw material for the production of stainless steel. PA1 4. Since, in the alkaline digestion of the slag rich in chromium oxide, the quantity of inert material (MgO, Al.sub.2 O.sub.3, FeO and Fe.sub.2 O.sub.3) reintroduced is small compared with the quantity of circulated leaning agent and since only that quantity of inert material introduced by the slag rich in chromium oxide has to be removed from the circuit, both the leaning agent and the inert material pass repeatedly through the digestion process. This multiple digestion of the same material results in a high extraction of Cr from the raw material used and hence in low Cr.sub.2 O.sub.3 contents in the small quantity of residue removed from the circuit. PA1 5. Whereas, where chrome ore is used, the Cr.sub.2 O.sub.3 to be digested is bound into the thermally and chemically resistant structure of the spinel which can only be digested under the most rigorous conditions, the chromium oxide slag produced in the described manner can surprisingly be digested with far less effort. This is also beneficial to the complete reaction of all the chromium present in the chromium oxide slag to sodium chromate. PA1 6. Surprisingly, however, the time required for digestion of the chromium-containing starting material can also be considerably shortened by replacing chromite with the chromium oxide slag typical of the process according to the invention in the oxidizing alkaline digestion process. This surprising effect provides for considerably better volume/time utilization of the digestion furnaces and is technically advantageous. PA1 7. The absence of aluminum from the digestion mixture enables the roasted material to be subjected to alkaline leaching so that all the sodium may readily be recovered as sodium bicarbonate.
More recently proposed processes, for example oxidation in the melt, have done little to eliminate these disadvantages. In addition, they impose exacting demands on the furnace material so that these problems have never been completely solved.
The accumulation of large quantities of residue also cannot be avoided by digestion with potassium hydroxide or potassium carbonate; to the contrary, voluminous aluminum hydroxide precipitation sludges are obtained.
The digestion of chromium ore with acid does not exceed 90% either. Iron and aluminum accumulate in the form of impure and hence worthless sulfates or ammonium double sulfates.
The chloridizing of chromium ore to form basically separable Fe(III) chloride and chromyl chloride is accompanied by extensive chloridizing of the secondary constituents and hence is a burden at the working up stage due to the accumulation of solutions of the metal chlorides.
Numerous attempts have also been made to use ferrochromium for the production of chromium chemicals by oxidation with air and with electrical current or chlorine or sulfuric acid. Despite a possible increase in the yield of chromium to 96% of the theoretical, none of these processes is capable of easing or even solving the problem of waste, particularly iron in the form of worthless iron hydroxides.
The only industrially practiced method mentioned above for the production of sodium dichromate as the most important starting material for all chromium chemicals essentially comprises three stages, namely:
In addition to chromite and sodium alkalis, particularly sodium carbonate, substances which are intended to maintain the porosity of the furnace charge during the digestion process ("leaning agents") are added to the burden. Porosity is necessary for forming a sufficiently large surface for the reaction with oxygen. The yield of chromium where chromite is used is between 74 and 90% of the chromium present in the chromium ore, depending on its composition.
The soluble monochromate is removed by filtration, mainly through drum filters, after cooling and leaching at a pH value adjusted by addition of acids or dichromate solution. The insoluble residue is repeatedly leached to reduce the content of Cr(VI). Part of the residue may be dried so that it may be reintroduced as leaning agent into the furnace burden.
The residue remaining is subjected to a reduction process in order to insolubilize the Cr(VI) remaining. This is done by treatment with reducing agents, for example Fe(II) sulfate (see also Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Weinheim, 1986).
MgO, Fe.sub.2 O.sub.3 and Al.sub.2 O.sub.3 participate to a very limited extent, if at all, in the digestion process. Nevertheless, they are
The SiO.sub.2 introduced and the aluminum oxide partly take place in the digestion process by reaction with the alkali carbonate (soda consumption by binding into the residue as aluminosilicate).
Accordingly, the problem addressed by the present invention was to provide a process for the production of sodium dichromate which